1. Field of the Invention
The present invention relates to both a porous mold used in pressure slip casting and a method of making the porous mold.
2. Description of the Prior Art
In the pressure casting process, the mold is required to have both passages for draining the water, which is forced from the molding surface of the mold into a porous layer, to the outside of the mold during high-pressure slip casting, and passages for supplying compressed air into the porous layer to spurt water or air from the molding surface through the porous layer when the cast product is to be removed from the mold.
Therefore, the mold used in the prior art is equipped with passages formed inside or at the rear surface of the porous layer to have communications with the outside of the mold.
Before entering into a detailed description of the present invention, cursory review of the prior art will be made in the following with reference to the accompanying drawings. FIG. 1 shows a mold which has channels 2 formed in the rear surface of a porous layer 1 as the passages. In a mold shown in FIG. 2, the channels 2 are formed in the porous layer 1 in parallel with the molding surface of the mold. In a mold shown in FIG. 3, the channels 2 extend into the porous layer 1 in relation perpendicular to the molding surface of the mold. In any of these examples, the porous layer 1 has its rear surface backed with a backing material 3.
These channels are so accurately pitched and spaced from the molding surface of the mold that the water and air may be spurted evenly from the molding surface when the cast product is to be removed from the mold. As a result, the channels raise difficulties in making the porous mold. If the porous layer is made thinner, the water and air cannot be injected evenly from the inner surface of the porous layer, i.e., the molding surface of the mold, unless the channels have a smaller pitch. However, it is difficult to form the channels at a small interval. This difficulty raises a defect that the porous layer has to be made thicker than necessary.
In order to overcome these defects, various molds have been proposed by Japanese Patent Laid-Open No. 17811/1973 and Japanese Patent Publication No. 14451/1981. According to the process disclosed in Japanese Patent Laid-Open No. 17811/1973, a two-layered structure is made by coating a mold of known plastic foam with a known porous material such as gypsum or the like. However, this process cannot control the pore diameter of the surface porous layer accurately but makes the surface porous layer excessively thin. From the restriction to the material of a coarse porous layer, moreover, this process is remarkably difficult to provide a mold for forming large-sized ceramic articles having a complicated shape.
Japanese Patent Publication No. 14451/1981 discloses a porous mold having a fine porous layer and a coarse porous layer integrated with each other by press working and subsequent sintering. The porous layers are formed from a mixture of resin powder and filler powder, the fine porous layer has a thickness of 0.3 to 5 mm as a surface layer.
The pressing working, however, makes it remarkably difficult to make a large-sized mold having a complicated shape. It is also remarkably difficult to control the pore diameter of the surface porous layer which plays an important role in the pressure slip casting process. Because of the sintering, moreover, dispersions may be incorporated into the sizing accuracy, strength and pore diameter so that the process is not appropriate for making the mold for pressure casting of the large-sized ceramic articles having the complicated shape.
There can be conceived another process for making a mold by merely adhering a surface fine porous layer prepared in advance and a rear coarse porous layer also prepared in advance to each other by means of an adhesive which is applied in a linear or net pattern having appropriate interval and width. In making the large-sized and complicated mold, however, the adhesive may have a tendency to slip the surface fine porous layer and the rear coarse porous layer relative to each other. Then, the adhesive applied portion is so locally widended as to block the water and air spurting from the molding surface of the mold, when the cast product is to be removed from the mold, so that the conceived process cannot be used to make a practical mold.
According to another process conceivable, the coarse porous layer consisting of a mixture of a liquid adhesive and a filler of appropriate granularity is adhered by a pressing or stamping method to the rear surface of a surface porous layer prepared in advance. Because of a low adhesion between the surface porous layer and the coarse porous layer, however, this process cannot make a practical mold capable of standing an actual use.